1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display device having optimized characteristic parameters, and thus having improved display characteristics and increased cell gap margin.
2. Description of the Prior Art
Generally, liquid crystal displays are divided into a transmissive liquid crystal display using backlight as a light source, a reflective liquid crystal display using natural light without backlight, and a transflective liquid crystal display, which selectively uses backlight or natural light according to circumstance.
Among such display devices, the reflective liquid crystal display requiring no backlight is useful for portable display systems, since it can have low consumption power, thin thickness and lightweight. As the market of mobile phones and portable devices is extended, a demand for the reflective liquid crystal display is gradually increased.
This reflective liquid crystal display comprises a lower substrate, a reflective electrode, a lower alignment film, a liquid crystal layer, an upper alignment film, a transparent electrode, a color filter, an upper substrate, a phase compensation film, and a polarizer, which are successively stacked on top of each other.
Moreover, the liquid crystal displays can be divided into TN (Twisted nematic) mode, GH (Guest Host) mode, ECB (Electrically Controlled Birefringence) mode, and OCB (Optically Compensated Birefringence) mode, according to their operation mode.
The TN liquid crystal display is currently widely used in computers and measurement devices, but has the problem of slow response speed. The ECB liquid crystal display is a mode of inducing a change in light transmittance by a change in birefringence of liquid crystal cells, and its typical example includes a HAN (Hybrid-Aligned Nematic) mode. The HAN mode liquid crystal display is actively studied since it has advantages in that it is operated at relatively low electric power and has fast operation speed.
Hereinafter, the structure and display characteristics of the reflective liquid crystal display will be briefly described.
FIG. 1 is a cross-sectional view, which schematically shows a reflective liquid crystal display according to the prior art. As shown in FIG. 1, the reflective liquid crystal display according to the prior art comprises a lower substrate 1 comprising a reflective electrode 2 and a lower alignment film 3, and an upper substrate 4 comprising a color substrate 2 and a upper alignment film 6 and disposed opposite to the lower substrate 1 while sandwiching a liquid crystal layer 10 therebetween. And on the outer surface of the upper substrate 4, a phase compensation film 7, such as a λ/4 film, and a polarizer 8, are successively adhered. The reflective electrode 2 has a rugged portion on its surface, which is formed by a lithography or holography process.
In this reflective liquid crystal display, upon no voltage application, light which was linearly polarized by passage through the polarizer is converted into circularly polarized light, such as left-circularly-polarized light, by passage through the phase compensation film. The circularly polarized light is converted into linearly polarized light by passage through the liquid crystal layer and reflected in the reflective electrode. Then, the linearly polarized light reflected in the reflective electrode is converted into left-circularly-polarized light by passage through the liquid crystal layer, after which it is passed through the phase compensation film and thus converted into linearly polarized light whose polarizing direction is parallel to the polarizing axis of the polarizer. Then, the linearly polarized light is passed through the polarizer to achieve a white state.
Moreover, in the reflective liquid crystal display, upon voltage application, light which was converted into left-circularly-polarized light by passage through the phase compensation film is passed through the liquid crystal layer intact and reflected in the reflective electrode to convert it into right-circularly-polarized light. This right-circularly-polarized light is passed through the liquid crystal layer and the phase compensation film to convert it into linearly polarized light whose polarizing direction is perpendicular to the polarization axis of the polarizer. Thus, the linearly polarized light cannot be passed through the polarizer so that a dark state is achieved.
In the reflective liquid crystal display, good display characteristics vary on how to optimize a characteristic value of the respective elements as described above. In other words, for an efficient increase in reflectance in the reflective liquid crystal display, the angle of the absorption axis or transmission axis of the polarizer, the optical characteristics of the phase compensation film, the thickness (d) of the liquid crystal layer, the phase delay value (dΔn) of the liquid crystal layer, the twist angle of liquid crystal molecules, the alignment angle of the alignment film, and the characteristics of the reflective film, etc., must be optimized.
For example, in the prior reflective liquid crystal display, good display characteristics can be obtained when one or two λ/4 films 7 are used, the alignment angle of the upper alignment film 6 is perpendicular to that of the lower alignment film, and the transmission axis of the polarizer 8 makes a 20° degree with the alignment angle of the upper alignment film 6 while making a 45° angle with the optical axis angle of the λ/4 film 7. In this case, the twist angle of liquid crystals and the phase delay value of the liquid crystal layer are controlled to about 63 to 80°, and 0.20 to 0.27 μm, respectively.
However, in the prior reflective liquid crystal display, if two λ/4 films are used for the optimization of a cell design, a reduction in reflectivity will be caused, whereas if one λ/4 film is used, a function of providing a phase difference of λ/4 over a wide range of visible light wavelengths will not sufficiently performed so that bad display characteristics will be caused.
In addition, in the prior reflective liquid crystal display, cell gap provided for obtaining good display characteristics is extremely small, and thus, yield in an actual process for the production of the prior reflective crystal display is disadvantageously reduced.